1. Field of the Invention
This invention relates generally to surface treatments for sterilization, cleanliness and microbial growth reduction and more particularly to a method of treating surfaces with reactive silanol which become self-sterilizing and which resist microbial growth without the use of biocides, and for the production of articles exhibiting microbial-resistant surface properties.
2. Description of Related Art
Siloxane coatings applied as reactive silanols have been granted three US patents known to applicants: Schutt/Gedeon, U.S. Pat. No. 5,929,159, Oligomeric silicon coating compositions, articles coated therewith and method for forming coating composition and coated articles based thereon; Schutt, U.S. Pat. No. 6,432,191, Silane-based, coating compositions, coated articles obtained there from and methods of using same and Schutt/Gedeon/Stanich, U.S. Pat. No. 6,451,382, Method for improving heat efficiency using silane coatings and coated articles produced thereby the entire disclosures of which are incorporated herein, in their entireties, by reference thereto.
The content of matter formulas described in these patents and any current or future derivative formulas for reactive silanols where such materials are applied using the methods defined herein for the purposes claimed herein are incorporated by reference. While not wishing to be bound by the following formulae provided for information, examples of reactive silanol compositions as described in the referenced Schutt et. al. patents are any coating, polish, primer, penetrant, sealer, or surface modification treatment comprised of an aqueous or non-aqueous dispersion of the partial condensate of monomethyl or monethyl silanol (by hydrolysis of monomethyl or monethyl alkoxysilane) alone or in admixture with minor amounts of other silanols, e.g., gamma-glycidyloxy silanol, phenyl silanol, etc, wherein the dispersions contain divalent metal cations, e.g., Ca+2, alcohol or water dispersants, and which may optionally contain film-enhancing additives such as, but not limited to, hydrolysis catalysts like acetic acid, ethylene glycol ether co-solvents, silicates or hydrolyzed silicates, solid or water-soluble pigments, gellation inhibitors like chromium acetate hydroxides, or metal alcholates of the of formula (2):M(OR3)m  (2)                where M is a metal valence 2, 3 or 4, or mixture of two or more such metals;        R represents a lower alkyl group; and,        m represents a number or 2, 3 or 4;or any reactive silanol pre-catalyzed (hydrolyzed) by adding water and additives to silanes and inducted for at least five, but not more than 20 minutes, and then diluted with solvent such as, but not limited to, lower alkanols such as isopropyl and ethyl alcohol to inhibit further polycondensation and cross-linking so as to be subsequently applied as 1-part reactive silanol that can be applied to a surface by spraying, brushing, or wiping; and which then optionally can be cross-linked into a polysolixane film by applying water and, preferably, water acidified with acetic acid or a mineral acid such as boric acid or other condensation additive where that water or aqueous mixture is mechanically buffed into the silanol layer using a wiping cloth, preferably, a microfiber polishing cloth or mechanical buffing wheel or similar device.        
Silicon-based or silicon-containing coatings and penetrants that can be applied and cured at ambient temperatures include silanes (typically alkylalkoxysilanes or alkyltrialkoxysilanes), siloxanes (typically oligomerous alkylalkoxysiloxanes or silsesquioxanes), silicates (including ethyl silicates, sodium silicates, and potassium silicates), methyl siliconates, blends of the above, and hybrid organic-inorganic paints and coatings including silicone alkyds, epoxy-siloxane coatings and acylic-siloxane coatings. In a recent symposium, a good history of recent developments was given by N. Andrew Greig of Arlington, Va.; “A Brief Overview of Reactive Silanes and other Siloxane Coatings as Corrosion Preventatives”.
In the early 1970s, Harold A. Clark of Dow Corning Corporation patented a variety of siloxane systems for lens coatings, fire-retardant binders for fire insulation, and a new variety of paints (see U.S. Pat. Nos. 3,944,702, 3,976,497, and related patents). Clark's invention involved generating RSi(OH)3 silanols in situ by adding trialkoxysilanes in an isopropyl alcohol-water carrier to an acidic dispersion of colloidal silica (Arkles, 607). This resulting sol condenses into a siloxanol polymer gel forming Si—O—Si chains that further cures to form a hard, adherent layer of silsesquioxanes (RsiO2/3). Clark created paint coatings by adding a variety of pigments to form flame-resistant paints and high-gloss enamels, to name a few.
When the Clark patents from 1976 expired, Dr. John Schutt of NASA Goddard and Tony Gedeon developed a new siloxane approach to overcome a weakness in the original Dow patents. Specifically, their U.S. Pat. No. 5,929,159 in July 1999 claimed that use of colloidal silica, “especially when used in or near the amounts contemplated by the above Dow Corning (Clark) patents, renders the coatings porous or microporous and drastically reduces the corrosion resistance of the coatings.” Their approach was to replace colloidal silica with divalent cations, particularly, Ca+2.
Again quoting from the '159 patent:
“Generally, when the silicon atom is both trifunctionally and quadrifunctionally hydroxylated, the resulting siloxane network accommodates minimally the passage of water vapor and in some circumstances also the passage of water as well as oxygen. Because of this property, bonding resulting from the hydroxylation at a metallic interface is incomplete and corrosion can occur. The present coating compositions better utilize the reactivity of the silanol moiety with substrate oxy and hydroxy species and promote the formation of a contiguous interfacial layer unaffected by surface and bulk diffusion of water, water vapor and oxygen. This is accomplished, at least in part, by replacing all or most of the colloidal silica in formulations of the type described in the Dow Corning (Clark) patents mentioned above with divalent metal (M+2) ions, such as, for example, Cu+2, Zn+2, Ca+2 Co+2, and Mn+2.
Other objectives of the new coating cited in this '159 patent include:
                to provide abrasion resistant coating compositions suitable for metallic and non-metallic surfaces.        to provide transparent, glass-like abrasion-resistant and corrosion resistant coating compositions as well as coated articles.        to provide such improved coating compositions as aqueous formulations with acceptable volatile organic component (VOC) levels and, therefore, environmentally acceptable.        to provide such coating compositions which may be prepared easily and economically and are easy to apply to various types of substrates.        to develop a coating composition suitable for coating marine surfaces, such as aluminum boat hulls, to render the surfaces corrosion resistant in a salt water environment.        
The resultant condensed organic-inorganic hybrid layer is thin (5μ-1 mil), transparent, and hard (pencil hardness 11H). Because the molecular size of the silanols before they cure into siloxane oligomers is so small, the sol penetrates pores in the substrate to achieve better sealing and bonding. The substrate can be a metal, non-metal, or an organic coating.
U.S. Pat. No. 6,451,382 to Schutt, Gedeon, and Stanich identifies that cured siloxane coatings inhibit molds, fungus, and bacteria by being hydrophobic and absent nutrients to support organic growth, and discloses that thin siloxane coatings are preferred for enhancing heat transfer, but does not link an observed antimicrobial effect to the nature of the film formed.
U.S. Pat. No. 5,954,869 notes that the antimicrobial properties of commercially available organosilanes such as the antimicrobial Dow Corning 5700 (Dow Corning Corporation, Midland, Mich.) are well known, but are impractical because of the instability of such compounds due to undesirable self-hydrolyzation. That is, such compounds are unstable in water. It further notes that quaternary ammonium silicon compounds also have been employed to sterilize or disinfect many surfaces, but that their employment is still limited because of their toxicity often as a result of the solvent system used to deliver the compound, the necessity for a solvent solution (for instance, Dow Corning antimicrobial agents contain 50% methanol), short term stability (stability of aqueous silane solutions varies from hours to several weeks only) and poor water solubility.
The present invention recognizes the discovery that, as oligomeric siloxane coatings polymerize from linear reactive polysilanols, the silicon atoms rotate to allow the larger organic groups upward mobility to the surface of the coating. Although not wishing to be bound by any theory of operation, this physical structure creates a permanently electron-deficient surface that inhibits growth of bacteria, molds, fungus, and algae, that inhibits proteins that some bacteria elaborate in order to attach to a surface and thrive, and may be responsible for killing microorganisms. A simplified model of the cured structure is provided.
This patent discloses water-stabilized organosilane compounds formed by mixing an organosilane, optionally having a nonhydrolyzable organic group, but having one or more hydrolyzable groups, with a polyol containing at least two hydroxy groups such as a carbohydrate, wherein at least any two of the hydroxy groups are separated by no more than two intervening atoms and methods for using the same to treat surfaces and articles by contacting a substrate with the product.
The present invention overcomes the limitations of these and other silicon-based antimicrobial treatments. The reactive silanols as cited herein are stable by being packaged in separate containers for mixing and hydrolyzation when needed for coating or by having a partially catalyzed silanol cured by post-coating application of water or water and curing agent and promoting cross-linking by mechanical rubbing. Under the methods described herein, the toxic alcohol emissions can be controlled during hydrolyzation to form reactive silanols and by controlling ventilation during application. Fresh formation of linear silanols from hydrolyzing tailored blends of preferentially tri-functional organosilanes maximizes the surface bonding reaction of hydroxy moieties with substrate hydroxides and oxides.
This method of formulation and mixing of reactive silanols, combined with the surface cleaning and application methods described herein allows surface treatment by standard paint application techniques or by dipping articles into reactive silanol sols and film bonding and curing at ambient temperatures. The resulting siloxane structure is cross-linked, insoluble in water, oil, solvents, and most acids and produces a thermodynamically stable antimicrobial, antifungal surface without resorting to additives. Further, the cured siloxane neither emits gases nor leaches toxins, nor is it toxic as a solid.